Due to the recent growing demand for energy, new power plant boilers, petroleum refining and petrochemical plants have been built. An austenitic stainless steel to be used in these manufacturing heating furnace pipes and the like, for use in those facilities is required to have not only excellent corrosion resistance but also excellent high temperature strength.
In such a technological background, for example, the Non-Patent Document 1 proposes a highly corrosion resistant austenitic stainless steel, having a reduced content of C together with N which is set at a level within a specified range, and containing Nb as a C-fixing element at a level within a specified range, thereby having excellent stress corrosion cracking resistance and high temperature strength, and showing no sensitizing even after a long period of aging without post heat treatment after welding.
Concerning the cracking in the Heat Affected Zone (hereinafter referred to as “HAZ”) of the austenitic stainless steel which contains C-fixing elements after welding, the Non-Patent Document 2 declares that the carbide dissolution in welding thermal cycles and reheating to the M23C6 precipitation temperature in the subsequent cycles lead to the formation of a sensitizing region, resulting in an intergranular corrosion cracking called “knife line attack”.
Further, as a result of detailed examinations using austenitic stainless steels containing Nb and C at high concentrations, the Non-Patent Document 3 and the Non-Patent Document 4 declare that the fusion of low melting point compounds, such as NbC and/or the Laves phase that has precipitated on the grain boundaries, causes liquation cracking in the HAZ. Therefore, they recommend that the precipitation of such low melting point compounds on the grain boundaries should be suppressed in order to prevent liquation cracking in the HAZ.
On the other hand, in the Non-Patent Document 5, it is pointed out that the weld zone of the 18% Cr-8% Ni type austenitic stainless heat resistant steels, undergo intergranular cracking in the HAZ after a long period of heating.
The Patent Document 1 discloses a stainless steel in which the C-fixing element is utilized. More concretely, it discloses a “stainless steel highly resistant to intergranular corrosion and intergranular stress corrosion cracking” having a specified chemical composition with Nb/C≧4 and N/C≧5. In the description that follows, “stress corrosion cracking” is referred to as “SCC”.
Further, the Patent Document 2 discloses an “austenitic stainless steel containing N for use at high temperatures”. More concretely, it discloses an “austenitic stainless steel containing N, which is excellent in sulfidation resistance and SCC resistance and is suited for use in a high temperature environment of 350° C. or higher where Cl− and S coexist” as resulting from the achievement of the sulfidation resistance under high temperature and high pressure conditions by an increased Cr content, improvement in chloride SCC resistance by the combined effect of increases in Cr content and Ni content and a decrease in C content and, further, the enhancement of polythionic acid SCC resistance by a reduction in C content, if necessary together with incorporation of Nb.
Patent Document 1: JP 50-67215A
Patent Document 2: JP 60-224764A
Non-Patent Document 1: Takeo Kudo et al., Sumitomo Metals, 38 (1986), p. 190
Non-Patent Document 2: Kazutoshi Nishimoto et al., Sutenresuko no Yosetsu (Welding of Stainless Steel) (2000), p. 114 [Sanpo Publications, Inc.]
Non-Patent Document 3: Yoshikuni Nakao et al., Journal of the JWS, Vol. 51 (1982), No. 1, p. 64
Non-Patent Document 4: Yoshikuni Nakao et al., Journal of the JWS, Vol. 51 (1982), No. 12, p. 989
Non-Patent Document 5: R. N. Younger et al.: Journal of the Iron and Steel Institute, October (1960), p. 188